Definition and Significance of Innate Immunity
Innate immunity is a genetically determined, inherited resistance of individuals of a given species to a specific antigen. It is driven by the biological characteristics of the organism itself, the properties of the antigen, and the nature of their interaction.
Understanding this topic is critical: immunology is built upon interconnected domains, and without clearly distinguishing the innate and adaptive branches, it is impossible to master subsequent topics such as antigen presentation, types of hypersensitivity, and vaccinology.
Features of Innate Immunity
- Activated almost instantly in response to microbial invasion and the damage of host cells; its cells constantly circulate in tissues.
- Repeated exposure to an antigen elicits a response of the same magnitude — immunological memory is absent.
- Recognizes conserved microbial structures (e.g., lipopolysaccharide of Gram-negative bacteria) and altered host structures.
- Receptors (PRRs, including Toll-like receptors) have a limited ability to distinguish between "self" and "non-self" — immunity is non-specific.
- The repertoire of receptors and recognized patterns is limited.
Factors and Functions of Innate Immunity
Factors are divided into three groups:
- Physicochemical barriers: skin and mucosal epithelium, antimicrobial molecules.
- Cellular component: phagocytes (macrophages, neutrophils), dendritic cells, mast cells, basophils, eosinophils, natural killers (NK), innate lymphoid cells (ILCs).
- Humoral component: complement system proteins, acute-phase proteins, defensins, lectins.
Main functions: destroy the pathogen; contain the infection while adaptive immunity develops; activate the adaptive response via antigen presentation.
Types of Inflammation and Execution of the Innate Response
The innate immune response depends on the type of pathogen and is executed through the activation of corresponding ILCs and a cytokine cascade:
- Type 1 — intracellular pathogens: ILC1s produce IFN-γ and TNF-α, activating M1 macrophages and the production of reactive oxygen species.
- Type 2 — parasites (helminths): ILC2s trigger an eosinophilic response and the production of IL-4, IL-5, and IL-13.
- Type 3 — extracellular bacteria and fungi: ILC3s synthesize IL-17 and IL-22, recruiting neutrophils.
Acquired (Adaptive) Immunity
Acquired immunity is resistance to an antigen formed during ontogeny as a result of the organism's interaction with that antigen (via infection or vaccination).
Key features:
- High specificity and the ability to recognize a vast number of antigens due to the diversity of BCR and TCR receptors.
- Clonal expansion: a cell with a unique specificity proliferates and forms a clone after recognizing the antigen.
- Forms slowly, requiring time for activation.
- Presence of immunological memory — long-lived memory cells providing a rapid and robust secondary response.
- Tolerance to self-antigens; its breakdown leads to autoimmune diseases.
Cells and Execution of the Adaptive Response
Lymphocytes play a key role:
- B lymphocytes provide humoral immunity through antibody production.
- T lymphocytes mediate cellular immunity: cytotoxic T cells (outdated term: T killers) destroy infected cells; T helpers coordinate the response; regulatory T cells maintain tolerance.
A dendritic cell that has captured an antigen migrates to a lymph node and presents it to a naive T lymphocyte via three signals. Depending on the cytokine microenvironment, the naive cell differentiates into:
- Th1 — targets intracellular pathogens, activates macrophages and NK cells.
- Th2 — targets helminths, activates eosinophils.
- Th17 — targets extracellular bacteria and fungi, recruits neutrophils.
Comparison of Innate and Adaptive Immunity
- Receptor specificity: innate — to conserved patterns; adaptive — to virtually any molecules.
- Receptor diversity: limited / virtually unlimited.
- Memory: absent / pronounced.
- Reaction to "self": possible / blocked by tolerance.
- Humoral factors: complement, lectins / antibodies.
- Key cells: phagocytes, DCs, NKs, mast cells / B and T lymphocytes.
- Speed: instantaneous / delayed.
Adaptive immunity alone cannot eliminate a pathogen (except for cytotoxic T cells) — it directs and enhances innate responses, making them specific.
Key points
- Innate immunity reacts instantly, non-specifically, and does not form memory.
- Adaptive immunity is highly specific, possesses memory, and exhibits tolerance to self-antigens.
- PAMP recognition is carried out via PRRs; adaptive cells use BCRs and TCRs.
- The type of inflammation (1, 2, 3) is determined by the nature of the pathogen and the type of activated ILCs and Th cells.
- Dendritic cells are the key link between innate and adaptive immunity.
- Th1 fights intracellular pathogens, Th2 fights helminths, and Th17 fights extracellular bacteria and fungi.
- The correct term for cytotoxic lymphocytes is cytotoxic T cells, not "T killers".
- Breakdown of adaptive immune tolerance leads to autoimmune diseases.